This invention relates to an improved wastewater treatment method and means through application of wastewater on to plants. Specifically, the invention relates to the utilization of a controlled culture of water hyacinth and/or other species of the Pontederiaceae family, in shallow basins for treating domestic, agricultural, and industrial wastewaters, and the like.
Culture of hyacinth (Pontederiaceae) in shallow, earthen basins for treatment of various wastewaters commenced in the mid-1970's. Some early systems are presently in operation. An engineering assessment of this process revealed that adequate data is available for system design, although much remains to be learned about the function and management of the same. See Aquaculture Systems for Wastewater Treatment-An Engineering Assessment. U.S. Environmental Protection Agency Report 430/9-80-007, June, 1980, pp. 63-80.
Some states have now formulated criteria for hyacinth basins. A typical criteria might be the requirement of multiple rectangular basins, each of about one acre provided with multiple inlets and outlets. A constant inflow rate is specified. Basins have a hydraulic detention of 3-5 days and are operated at depths of about three feet. Basins must annually be taken out of service, drained, dried, and excess sediment removed.
Hyacinth do not survive in saline water and are killed by freezing temperatures. As a result, hyacinth culture in uncovered basins year-round is only feasible in southern California, Florida and Texas within the continental United States. Greenhouse culture is necessary elsewhere, and such culture will mostly be below latitude 32 degrees North. As is known, other species of Pontederiaceae, e.g. Pontederia cordata, can be cultured in greenhouses in cold temperature climates.
Properly loaded, hyacinth basins are capable of producing high quality effluents, low in oxygen-demanding matter, suspended particles, and total nitrogen. Phosphorus, at best, is only removed by some 30-40%, but the clear effluent facilitates chemical removal of phosphorus by precipitation. Significant advantages of hyacinth over conventional bacterial-based systems include:
(1) consistency of high quality effluent production; PA1 (2) lower capital and operating costs; PA1 (3) better energy utilization; PA1 (4) they are more environmentally acceptable; and PA1 (5) in particular, their capability of returning nitrogen back into the atmosphere in gaseous form is preferable.
Hyacinth grow rapidly and absorb soluble nutrients, particularly nitrogen. Except for carbon dioxide obtained from air, biomass is derived from wastewater nutrients. One management approach is to continually harvest the hyacinth with the harvest rate matched with optimum plant productivity to maximize nutrient uptake and removal. Such a scheme, because of harvesting and processing costs, must result in salable products. The market potential of waste-grown hyacinth is thought to be greater for dairy and beef cattle rations than for methane generation, compost or other products. As a result, commercial hyacinth production implies a warm climate, extensive culture area, large wastewater flows and a nearby product market, a combination of requirements that exist only in southern Florida within the continental United States.
Systems used solely for treatment practice minimal harvesting of plants along basin edges to preclude sudd formation, with complete removal of hyacinth at the time of basin cleaning only. The main features of a hyacinth basin are: an overstory of plant leaves about three feet high; a shallow aerobic zone with a dense, intertwined mat of roots; a facultative open water zone; and an anaerobic benthal sediment layer. Typically, most particulate sedimentation and carbonaceous oxygen demand reduction takes place in the upstream side portion of a basin.
A hyacinth leaf consists of a tubular petiole and a flattened blade or lamina (stems and leaves). Both stems and leaves have thousands of tiny openings (stoma) per square centimeter to serve for gas exchange (oxygen, carbon dioxide, and water vapor). Oxygen, either that absorbed from the atmosphere or resulting from photosynthesis, is transported, probably mostly by diffusion, through leaves, stems and submersed rhizomes into roots, and thence into surrounding waters. This mechanism in conjunction with the ability of roots to extract nutrients from the water, even when present in minute amounts, permits hyacinth to grow in anaerobic semi-solid organic sludges or in clean spring waters. Plant diffusion of oxygen into the shallow waters in the root zone is a key factor in the function of an aerobic hyacinth basin. Leaves shade the surface, retarding algae growth, and contribute to basin water stillness that enhances sedimentation. Typically, the stems and leaves of hyacinth, except for nutrient uptake, have no direct role in cleansing wastewaters.
It is known that the depth of root mat (root length) is related to nutrient levels, especially nitrogen, in wastewaters. Typically, roots extend about six inches below the surface. Root length may be up to eighteen inches in the lower ends of basins when effective nitrogen exhaustion has been attained. The oxygen resource of the root zone is mainly dependent upon that diffusing from roots and a small amount that diffuses into the water surface from the atmosphere. Up to about 100 pounds of BOD.sub.5 per acre each day can be imposed on a basin without exhausting available oxygen. This limited loading is a major constraint of the current systems due to space needs.
Because of the dominating presence of a large floating plant and diverse, extensive macrofauna populations, the complex ecosystems of aerobic hyacinth basins have to be viewed in an entirely different context than artificially limited environments of conventional treatment units strongly dominated by aerobic microbiota.
Populations of a hyacinth basin grow or decline in number according to food (organic loading or BOD.sub.5) available. Hydraulic flow control is the only practical method of assuring relative steady input of food. This requires an equalization basin. An alternative is to size basins on maximum expected organic and hydraulic loadings, as both can be excessive. Excessive hydraulic loading can cause system "breakthrough". Organic overload is damaging, both on a short-term and long-term basis. Replacement time for some macrofauna species is brief, but others, such as predaceous insect larvae, will be absent until laying time the following spring. Death of the numerous fish species, crayfish, etc., present requires restocking. The tenuous nature of available oxygen resources requires very careful control of food (organic) input.
Recognizing that nitrification and much removal of carbonaceous oxygen demand and suspended particles occur in the root mat, researchers have directed attention to techniques for bringing more wastewaters in contact with microbiota on hyacinth roots to improve the system. Techniques tested include; emplacement of overflow baffles perpendicular to flow; extensive recirculation; and compressed air injection.
Emplacement of closely spaced overflow baffles in full-sized hyacinth basins is impractical and a study involving recirculation of wastewaters fifteen times through a 7-foot deep basin resulted in only slightly improved effluent quality over that obtained from a once-through flow in the control unit.
Compressed air injection, aside from cost and inefficiency of oxygen transfer at shallow depth, has several detrimental effects. Some of these are:
(1) aerobiosis allows bacterial dissimilation of benthal debris;
(2) feed-back from aerobic microbial decomposition of benthal debris stimulates more unneeded bacterial biomass, requiring even more oxygen;
(3) habitat and carbon source required by nitrifiers are eliminated, thereby reducing system capability for total nitrogen extirpation;
(4) turbulence interferes with sedimentation;
(5) the valuable sediment storage function is removed;
(6) settled particles are resuspended;
(7) stored metals are freed from benthal sediment; and
(8) although sediment accumulation is severely reduced by continuous recycle back to the water column, clearance of basins at regular intervals is necessary to prevent sudd formation. This would entail removal of air distribution tubing from basins, obviously impractical and very expensive on a large scale.
One technique evaluated on a pilot-scale involved directed air injection via floating headers to effect horizontal flow within the root mat zone. This approach leaves the anaerobic benthal debris layer relatively undisturbed and near surface water turbulence probably has minimal adverse impact on sedimentation. Horizontal flow induced through the thick, entangled root mass was likely limited, but in spite of this, pronounced treatment efficiency and effectiveness was recorded, including enhanced total nitrogen removal. Surface flow induction using air release may be viable in very small basins but placement of headers and air lines at adequate intervals to assure surface water turbulence throughout a full-sized basin would be impractical. Removal of headers and air lines from a basin covered with a dense stand of mature hyacinth at cleaning time would be a most difficult and costly operation. See Community Waste Research at the Walt Disney World Resort Complex. Brochure-Walt Disney World Resort, Orlando, Fla., 1986.
Frost protection of citrus trees by water spray has been practiced for many years. Likewise, frost protection of hyacinth growing in open basins at Shreveport, La. has been evaluated. Screened effluent was sprayed onto plants at times when air temperatures were near or below freezing. Spraying, however, did not prevent hyacinth death.
U.S. Pat. No. 4,169,050, granted to Serfling and Mendola, and titled "Buoyant Contact Surface in Waste Treatment", involved employment of bottom-anchored, buoyant plastic ribbons for microorganism attachment in a greenhouse-covered 8-foot deep lagoon aerated by bottom-released compressed air. The lagoon was to be stocked with various faunal species and the surface used for culture of floating plants including duckweeds, hyacinth, and water fern, with plants being harvested and put to useful purposes.
Another feature of the system involved spraying water into the air for solar heating and cooling and humidity control, a technique commonly called "misting" that is widely practiced by greenhouse operators growing nursery plants or food crops. Incidental benefits of mist spraying mentioned were aeration, water movement, pest removal, and foliar-feeding of the plants.
A relatively small quantity of water was used in misting, but very clean water was required to preclude clogging of the tiny openings of the mist nozzles.
Drawbacks to current wastewater treatment utilizing floating plants then are the ineffective methods for enhancing root/biofilm contact as previously discussed, such as baffling recirculations and upwelling using compressed air. Thus, there is a need in the art for providing a wastewater treatment that enhances root/biofilm contact without unnecessarily disrupting the normal functions of the hyacinth wastewater treatment device. It, therefore, is an object of this invention to provide an improved water treatment that enhances root/biofilm contact; that enhances aerobic conditions for the increased effectiveness of the system as a whole; that reduces the cost of construction of facilities through enhanced effectiveness of the system as a result of increased system capacity; that utilizes and exploits the overstory of the floating plants thereby improving system efficiency and effectiveness and expanding treatment capacity; that does not disturb sediments and that does not require the complete removal and replacement of application/distribution devices once installed.